Pulse-width modulated systems are used to provide high efficiency amplification and transmission in applications that vary widely from low-power consumer audio devices, such as MP3 players, to high power data transmission circuits such as base station transmitters. High efficiency is achieved by minimizing power losses due to bias current within the output stages of an amplifier. For example, in audio applications, a class-D amplifier is used to switch the terminals of a loudspeaker between two supply voltages at a frequency greater than the bandwidth of the desired output signal. Here, high frequency switching energy is filtered by the characteristics of the load circuit, for example, the inductance of the loudspeaker. Similarly, in RF applications, a power amplifier (PA), is driven by a pulse-width modulated signal with a pulse frequency greater than the bandwidth of interest. Out-of-band energy is then filtered using an RF bandpass filter, such as a SAW filter. Because there is a minimal IR drop across the output stage of devices operating in a switched manner, as can be the case when PWM signals are used, dissipated power across the output stages of the devices are minimized and efficiency is improved.
The generation of high dynamic range PWM signals, however, poses a number of challenges. Because the amplitude of the signal is embedded within the timing of a pulse train, jitter and inaccuracies in edge transitions may lead to increased noise and decreased dynamic range. The sampled nature of a PWM system can lead to further reductions in dynamic range due to noise folding and aliasing. This is further exasperated when a PWM signal is upconverted to another frequency, for example, in an RF system. Moreover, the generation of aliasing, images and out of band noise potentially causes adjacent band interference, and is often attenuated using high order filtering.